It’s so easy to do. Just define in front matter an image and a background color. Instead of a color you can also use a pattern image. Have a look at the example with a background pattern.
Front Matter Code
WARNING: To make this work the value of background-color must be inbetween quotes.
header: image: "header_unsplash_2-970x.jpg" background-color: "#fabb00" caption: This is a caption for the header image with link caption_url: https://unsplash.com/
design background color header